THE DISCOVERY OF COMPLEX MOLECULES IN SPACE

Astrobiology: The Discovery of Complex Molecules in Space; Implications for Planet Formation and Life



Astrobiology: The Discovery of Complex Molecules in Space; Implications for Planet Formation and Life

Introduction

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The discovery of complex molecules in interstellar space in the late 1960s by Townes, Welch, and their collaborators destroyed the conventional wisdom of the day and created widespread interest in the chemistry of interstellar dust clouds. On page 1508 of this issue, Kaiser et al. heighten this interest by providing evidence that a new class of reactions - reactions between certain charge neutral precursors - may be a critical component of this chemistry.

In the 1960s it was recognized that self-shielding of dense ([10.sup.3] to [10.sup.6] [CM.sup.-3]) clouds against interstellar ultraviolet radiation permits a rich and complex chemistry to pervade these objects, but the mechanism for the formation of molecules at the ultralow cloud temperatures (T < 50 K) remained an enigma. This dilemma was solved in the mid-1970s by Herbst and Klemperer (3), who proposed a scheme of ion-molecule reactions, known to often possess nearly zero activation barriers, that nicely accounted for the principal observations. After the detection in dense clouds of two molecular ions, [HCO.sup.+] and [HNN.sup.+], by Snyder (4) and Thaddeus and collaborators (5), respectively, and the confirmation of these observations by microwave spectroscopy in the laboratory of Woods (6), a scheme featuring ion-molecule reactions driven by cosmic ray ionization of hydrogen followed by ion-electron dissociative recombination reactions became the paradigm for the chemistry of interstellar dust clouds. Furthermore, it became recognized that molecules were intimately involved as cooling agents in the critical processes of star formation and gravitational collapse of these objects.

More recent observations have made it clear that this picture is, however, incomplete. The detection of long-chain carbon-containing molecules like [HC.sub.5 N] in dark dust clouds by Avery and co-workers (7) revealed a class of molecules, the high abundance of which was not readily explained by the ion-molecule scheme. The discovery of larger interstellar molecules ([HC.sub.11 N], for example) along with pure-carbon-chain species like [C.sub.3] and [C.sub.5] (8) in circumstellar shells of dying carbon stars and possibly in cold clouds (9) likewise implied that another type of chemistry must be occurring, at least under some conditions. The tentative association of polycyclic aromatic hydrocarbon molecules with the endemic unidentified interstellar emission bands may constitute still another example (10). In summary, it now seems that much of the carbon chemistry of interstellar dust clouds is not readily explained within the context of the ion-molecule reaction scheme. This is a problem of considerable significance because carbon is the fourth most abundant element, is the critical element for life as we know it, and is most likely to be crucially involved in the nucleation of solid matter from the gaseous state.

Discussion

Until very recently, reactions between neutral species have not been considered relevant to interstellar cloud chemistry (except for the formation of molecular hydrogen, which is thought to ...